1. Field of the Invention
This invention relates to an endoscope apparatus equipped with an auto-iris device which is capable of changing the aperture size of stop means in response to information on the brightness of an object derived from the intensity of a signal output from a charge coupled device image sensor.
2. Description of Related Art
Some of endoscope apparatus in recent years have been designed so that a TV camera housing a charge coupled device image sensor (which is hereinafter referred simply to as a CCD or a solid-state image sensor) is attached to the connection of a non-flexible endoscope and an image inside a human body is displayed on a monitor for diagnosis and therapy. FIG. 1 shows the entire construction of the imaging system of such an endoscope apparatus. Reference numeral 1 denotes an endoscope (non-flexible endoscope); 2 a TV camera head; 3 a TV camera control unit (CCU); 4 a monitor; and 5 a light source device.
The endoscope 1 includes an objective lens 7 provided with a fixed stop 6, an image transmitting optical system composed of a plurality of relay lenses (three in this figure) 8, 9, and 10, and an observing optical system composed of an eyepiece 11. It further includes a light guide 12 composed of a fiber bundle juxtaposed with the objective lens 7 and the image transmitting optical system. The TV camera head 2 is equipped with an adapter lens 13, a CCD 14, and a signal cable 15 for transmitting an output signal from the CCD 14. The light source device 5 includes a source lamp 16, a source stop (light control means) 17, and a collector lens 18. The endoscope 1 is connected with the light source device 5 by a light guide cable 19.
The light guide cable 19 is such that its one end is connected to the light source device 5 and the other end is connected to the light guide 12 of the endoscope 1 through a connector 21 incorporating coupling lenses 20. Light emitted from the source lamp 16 travels through the source stop 17 and is collected by the collector lens 18 at the entrance end of a fiber bundle 22 encased in the light guide cable 19. The light emerges from the exit end of the fiber bundle 22 and is collected at the entrance end of the light guide 12 by the coupling lenses 20. Subsequently, the light is radiated from the exit end of the light guide 12 toward an object M.
Reflected light from the object M is incident on the objective lens 7 to form the image of the object behind the objective lens 7. The image is transmitted to the front of the eyepiece 11 by the image transmitting optical system while being formed in succession by the respective relay lenses. Light from the image emerges as nearly parallel beams from the eyepiece 11 toward the adapter lens 13 situated inside the TV camera head 2.
When the TV camera head 2, although it can be removed from the eyepiece section of the endoscope 1, is mounted thereto, the adapter lens 13 receives the light from the eyepiece 11 so that the image of the object M is formed on the CCD 14. The output signal from the CCD 14 is supplied through the signal cable 15 to the CCU 3. The CCU 3 has the functions of converting this signal into a particular signal (such as a television signal following the NTSC standard) which can be displayed on the monitor 4, and also of processing various signals when necessary. Furthermore, the CCU 3 is provided with the function of generating various control signals for controlling the entire system. A TV signal output from the CCU 3 is fed to the monitor 4, on which the image of the object M is displayed.
The CCU 3 is adapted to detect the brightness of the reflected light from the object M by making use of the output signal derived from the CCD 14 and to supply an automatic light control driving signal to the light source device 5 so that the reflected light has optimum brightness. With this signal, the source stop 17 is controlled and the amount of light irradiated on the object M is properly adjusted.
Such endoscope apparatus, chiefly used for surgical operation, are attended with intricate work in order that a doctor performs focusing and secures the range of observation during the operation. One of means for solving this problem is to mount an autofocus mechanism on the endoscope apparatus. However, in endoscopes, unlike the case of ordinary photographic cameras, the position of a part observed by a doctor is not necessarily limited to the center of the visual field, and thus it is difficult to bring the doctor's desired part for observation into an accurate autofocus state. In addition, it is also technically difficult to incorporate a focus detecting element in the endoscope with a very small diameter. Hence, in order to ensure the observation range of the endoscope, it is desirable to increase the depth of field and bring about a pan-focus state. Endoscopes are constructed so that the light guide composed of an optical fiber for light transmission is disposed therein and illuminating light is radiated from the distal end portion thereof. When a luminance Bk of the light source is constant, an illuminance E of an object surface is proportional to the square of a distance x between the distal end portion of the endoscope and the object M. Brightness thus changes with the distance x. In order to properly hold the illuminance E, irrespective of the distance x, it is desired that the luminance Bk can always be set to the optimum value (for example, the output which finally becomes 80IRE as a TV signal).
Thus, in the conventional endoscope apparatus, as shown in FIG. 1, the aperture of the stop 6 is fixed in a state where it is stopped down to some extent, and the illuminance E of the object surface varying with the distance x from the distal end portion of the endoscope 1 to the object M has been corrected in such a way that a filter, such as a neutral density (ND) filter, is disposed inside the light source device 5 to change the amount of light of the light source device 5, and thereby an illuminance E' of the image surface of the CCD 14 is made constant.
The relationship between the illuminance E' of the CCD 14 and the distance x in this case is shown in FIG. 2. In this diagram, the axis of ordinates is the illuminance E' of the CCD 14 and the axis of abscissas is the distance x. Curves A to C drawn by broken lines represent how the illuminance E' of the CCD 14 varies with the distance x in the case where the luminance Bk of the light source device 5 and the aperture of the stop 6 are constant.
Calling r the reflectance of the object and NA' the numerical aperture governed by the optical system, the image illuminance E' is given by EQU E' .infin.r Bk NA'.sup.2 /x.sup.2 ( 1)
Thus, in FIG. 2, the improvement of the luminance of the light source brings about a change in the luminance Bk and consequently, the illuminance E' changes as in the curves A to C. This graph shows that the source stop 17 shown in FIG. 1 is controlled so that the amount of illuminating light supplied by the source lamp 16 changes (namely, the luminance of the light source changes) and the illuminance E' varies between the curve A (small in the amount of light) and the curve C (larger in the amount of light) accordingly. In the range of object distances x2 to x3, the control of the amount of light by the source stop 17 is possible, and thus the image illuminance E' can be kept constant. A point a indicates the position where the amount of transmitted light of the source stop 17 is minimized, and when the object is closer (x&lt;x2), the image illuminance E' ceases to be controllable and increases. In contrast to this, a point c is the position of the maximum amount of transmitted light of the source stop 17 and when the object lies farther away, the image illuminance E' decreases. Although the curves of this graph may fluctuate because of the variation of reflectance of the object and the unevenness of light of the source lamp, it is here assumed, for simplicity, that such fluctuation is caused only by a change in luminance of the light source.
Here, it is assumed that the luminance Bk of the light source is constant on a curve B and the aperture of the stop 6 is set to .phi.1 so that the image illuminance E' of the object at a distance x1 comes to an appropriate image illuminance E'1. Then, the luminance Bk of the light source is controlled so that the illuminance E' becomes the constant value E'1 to shift along the solid line of the graph. In this case, within the region of the points a to c in which the illuminance E' is constant, observations will be made with exactly the same F-number (the aperture .phi.1 of the stop 6).
In the above-mentioned conventional endoscope apparatus, if the objective optical system is intended to be in a pan-focus state, the aperture of the stop 6 must be diminished and thus brightness becomes insufficient. Consequently, since the aperture .phi.1 of the stop 6 is determined in view of both the brightness and the depth of field, this system is not said to be advantageous to both. In recent years, however, an auto-iris device (hereinafter referred to as an AI device) in which the aperture of the stop can be changed by an electric signal has become compact in design and low in cost. Thus, the application of the AI device to the attachment TV camera for endoscopes shown in FIG. 1 is being discussed. If the AI device is mounted to the attachment TV camera for endoscopes, the endoscope system mentioned above will have two kinds of means for light control, including light control means with the light source device which is already provided. The AI device, which has a considerable effect on the depth of field as well as on brightness, involves the difficulty, depending on light control techniques, that brightness is sufficient but the depth of field is small, or brightness is improper although the depth of field is considerable. Consequently, the endoscope needs such light control means that a favorable observation range is ensured and at the same time, brightness remains unchanged.